Any control system having an operator interface can be susceptible to the operator coupling with the system while the operator is operating or controlling the system. For example, the response characteristics of the system can cause the operator to make inputs that are too large (e.g., over-controlling) and/or out of phase with the system response when attempting to operate or control the system during certain tasks. When the system includes an aircraft, this phenomenon often results in unwanted aircraft motion resulting from the pilot's control input and is referred to as an aircraft pilot coupling (APC) event. When the unwanted aircraft motion is oscillatory in nature, the APC event is often referred to as pilot-induced oscillation (PIO). APC events can degrade aircraft handling qualities and/or lead to aircraft control problems.
Over the years, aircraft designers have developed different criteria to predict the APC susceptibility of different designs. These criteria have included the bandwidth criterion, the Gibson criterion, the Open Loop Onset Parameter criterion, the Smith-Geddes criterion, the Neal-Smith criterion, and the Hess criterion. Although some of these criteria examine both the frequency and gain of the system, in many cases, they do not yield the desired accuracy and/or provide the desired insight into system performance. Additionally, many of these criteria are cumbersome, complex, and difficult to use.